"Colorless green ideas sleep furiously" is a sentence composed by Noam Chomsky in 1957 as an example of a sentence whose grammar is correct but whose meaning is nonsensical, however some might argue that Chomsky simply wasn't imaginative enough to put the sentence into a context which would give it meaning.[1] It was used to show inadequacy of the then-popular probabilistic models of grammar, and the need for more structured models.

The full passage says:

(1) Colorless green ideas sleep furiously.

(2) Furiously sleep ideas green colorless.

It is fair to assume that neither sentence (1) nor (2) (nor indeed any part of these sentences) had ever occurred in an English discourse. Hence, in any statistical model for grammaticalness, these sentences will be ruled out on identical grounds as equally "remote" from English. Yet (1), though nonsensical, is grammatical, while (2) is not.

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The sentence can be given meaning through polysemy. Both green and colorless have figurative meanings, which still make us able to interpret colorless as "nondescript" and green as "immature" or "environmentally-friendly". So the sentence can be construed as "nondescript immature ideas have violent nightmares", a phrase not unimaginable in poetry. In particular, the phrase can have legitimate meaning to bilingual English-Spanish speakers, for whom green can mean "newly-formed" and sleep can be used as a verb of nonexpression. An equivalent sentence in English would be "Newly formed bland ideas are unexpressible in an infuriating way." One meaning could be "unimaginative environmentalist ideas are unpopular".

The results have been published of a 1985 literary competition in which the contestants attempted to make Chomsky's sentence meaningful using not more than 100 words of prose or 14 lines of verse.

The game of cadavre exquis (1925) is a method for generating meaningless sentences. It was named after the first sentence generated, Le cadavre exquis boira le vin nouveau (the exquisite corpse will drink the new wine).

There are doubtless earlier examples of such sentences, possibly from the philosophy of language literature, but not necessarily uncontroversial ones, given that the focus has been mostly on borderline cases. For example, followers of logical positivism held that "metaphysical" (i.e. not empirically verifiable) statements are simply meaningless; e.g. Rudolph Carnap wrote an article where he quite literally claimed that almost every sentence from Heidegger was grammatically correct, yet meaningless. Of course, non-logical positivists disagreed with this.

Another example is "Time flies like an arrow. Fruit flies like a banana" which mixes syntactic confusion with semantic confusion and thus indicates the problems (again) of a purely syntactical approach to parsing natural language without semantic context.

The philosopher Bertrand Russell used the sentence "Quadruplicity drinks procrastination" to make a similar point; W.V. Quine took issue with him on the grounds that for a sentence to be false is nothing more than for it not to be true; and since quadruplicity doesn't drink anything, the sentence is simply false, not meaningless.

Examples like Tesnière's and Chomsky's are the least controversially nonsensical, and Chomsky's example remains by far the most famous.

Fernando Pereira of the University of Pennsylvania has fitted a simple statistical model to a corpus of newspaper text, and shown that under this model, "Furiously sleep green ideas colorless" is about 200,000 times less probable than "Colorless green ideas sleep furiously".
[2] (See also this post at Language Log.) This does not challenge Chomsky's (1965) point that sentences such as "colorless green ideas sleep furiously", despite being 100% grammatical, have a probability of production "empirically indistinguishable from zero". They will therefore be predicted to be at the ungrammatical end of the scale under any model of grammaticality based on probability of production, and this remains true whether or not ungrammatical sentences which contain the same word in a different order are even less likely to be produced. In fact, it might be questioned whether Pereira's model really is an accurate model of likelihood of production, and hence whether its prediction that the ungrammatical sentence is less probable than the grammatical sentence is correct. The statistical model defines a similarity metric, whereby sentences which are more like those within a corpus in certain respects are assigned higher values than sentences less alike. But whether such similarity translates into greater probability of production is an empirical question with no obvious answer — especially given the fact that the model is not directly sensitive to semantic (in)coherence. In other words, it is not clear how the model could be empirically tested as a model of production probability, rather than as a mere similarity metric.